Extraction of solid material is traditionally performed by standard techniques such as a Soxhlet extraction. The Soxhlet method proceeds by iterative percolation of the sample to be extracted with recondensed vapours of solvent. This method is one of the most used technique for extraction of organic contaminants for example. Since this process can be quite lengthy, new methods using microwave radiation have been developed.
For example, WO 2004/062766 A1 describes a microwave-accelerated extraction of components of solid samples. The method consists in heating up the solvent by conventional electrical heating, allowing the solvent to reflux over a glass tube containing the sample to be extracted. The sample to be extracted is at the same time subjected to microwaves. This allows the extraction of compounds which are only weakly polar or apolar such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dioxins, etc.
Another method for extracting organic compounds from a solid sample by microwaves is described in WO 97/44109 and consists in subjecting a sample in a solvent to microwave radiation such that the solvent can reflux and such that the organic compound can be extracted in a way similar to a Soxhlet extraction. In the method, the solid material is placed in a movable, porous container, which can be retracted out of the solvent.
Using a microwave assisted extraction process such as those described above presents the advantage that the whole extraction process is accelerated. However, it has been found that the moisture content of samples to be extracted is a defining parameter for the recovery yield, when carrying the methods of the prior art. For instance, Lopez Avila et al. (J. AOAC Int. 1996, 799, 1, 142-156) and McMillin et al. (Spectrosc. 1997, 13, 41-50) have reported that the recovery of polar pollutants is improved with moisture. Furthermore, Budzinski, H. et al. in Polycyclic Aromatic Compounds 1996, 9, 225-232 have reported that the maximum recovery for PAHs is obtained when the moisture content of the sample is between 20 to 100%, preferably about 30%. When using dried samples, the recovery yield drops dramatically to about 15%.
The prior art methods are therefore limited in terms of solvents choice, nature of the solid material to be extracted, moisture content etc.